Water treating method

ABSTRACT

The present invention relates to a water treatment process comprising adding ozone to raw water and filtering the raw water using an ozone-resistant membrane, wherein a concentration of ozone present in the water filtered through the ozone-resistant membrane is detected and an amount of ozone to be fed into raw water is continuously and automatically controlled so as to adjust an ozone concentration to a prescribed value.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/JP98/04980 which has an Internationalfiling date of Nov. 5, 1998, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to an advanced treatment process ofservice water and secondary treatment waste water, and to a treatment ofindustrial water or waste water. More particularly, it relates to amembrane filtration treatment process of water using ozone thatconstantly and effectively provides a filtrate with a certain highquality.

BACKGROUND ART

A typical conventional water purification process is a coagulationsedimentation process. This process comprises first adding chlorine orsodium hypochlorite to collected raw water, oxidizing iron or manganeseso as to make them insoluble and at the same time preventingmicroorganisms from increasing and from causing adverse effects in thepurification process, then adding a coagulant appropriate to coagulatesuspended substances, flocculating the coagulated substances forsedimentation separation, removing overflowed flocs by sand filtration,and cleansing the raw water. When the raw water contains few suspendedsubstances, it may be treated according to a process comprisingline-mixing coagulants followed by a sand filtration and a processcomprising floating suspended substances by pressure.

However, it is impossible to achieve the water purification levelrequired in the recent semiconductor industry and the protozoa removallevel required in waterworks only by the conventional coagulationsedimentation processes. As the quality of raw water to be treated inwaterworks has been debased year by year, there has been a demand, forexample, to remove colored elements and elements having a foul odor andtaste, to remove harmful organic substances such as agriculturalchemicals and environmental hormones, to remove protozoas such asCryptosporidium and Giardia, and to reduce chlorine disinfectionby-products such as trihalomethanes. For satisfying such demands,advanced treatment such as biological treatment, treatment with ozoneand treatment with activated carbon has been initiated.

Recently, membrane filtration processes using an ultrafiltration (UF)membrane and a microfiltration (MF) membrane have been adopted insmaller waterworks because of their advantages over the coagulationsedimentation process, such as a high ability to remove impurities,germs and protozoas sufficiently, high reliability and capability ofautomatic operation. Further, combinations of the membrane filtrationtreatment with the above advanced treatment are being studied.

However, the membrane filtration treatment requires regular cleansing ofchemicals since continuous filtration causes clogging of the membrane sothat a filtration flux gradually decreases. In order to reduce thefrequency of cleansing of chemicals to as low as possible, it isnecessary to carry out troublesome pretreatment such as coagulationsedimentation or to set the membrane filtration flux small. Therefore,its applicable field is limited by economic concerns. Further, accordingto the membrane filtration treatment, the protozoas contained in the rawwater such as Cryptosporidium are completely removed so that thefiltrate becomes safe. However, the condensed waste water produced bythe membrane filtration contains concentrated protozoas, and thereforerigid oversight is required for the disposal of the waste water.

In order to avoid the above drawbacks, for instance, U.S. Pat. No.5,271,830 and PCT International Patent Publication No. WO97/10893disclose processes for preventing a membrane from clogging by feedingozone upstream of the membrane and simultaneously improving waterquality. However, for achieving sufficient effects in such a treatment,the ozone must be supplied excessively in anticipation of the change inthe quality of the raw water. Therefore, these processes areeconomically disadvantageous. Further, the increase in the feedingamount of ozone unpreferably results in the formation of by-products andthere is an increased load on the activated carbon in the followingstep, i.e., the use of activated carbon is increased by reacting withthe ozone remaining in the filtrate.

On the other hand, a smaller feeding amount of ozone decreases themembrane filtration flux due to the clogging of the membrane. As aresult, the water quality is not sufficiently improved. Moreover, if theraw water quality is changed, the membrane filtration flux is greatlyfluctuated and the quality of the treated water is also fluctuated.Accordingly, it is difficult to ensure a certain amount of treated waterwith a certain quality all the time.

DETAILED DESCRIPTION OF THE INVENTION

An object of the present invention is to provide a process of membranefiltration treatment of water using ozone, wherein a filtrate having acertain high quality is obtained while a large filtration flux ismaintained regardless of the fluctuation in the quality of raw water.

Another object of the present invention is to provide a process ofmembrane filtration treatment of water with ozone, wherein an amount ofozone remaining in the filtrate is kept small so as to conduct thefollowing water treatment steps effectively.

A further object of the present invention is to provide a watertreatment process, wherein treated water with a high quality is obtainedusing a compact system and harmful wastes are not produced.

The water treatment process of the present invention comprises addingozone into raw water and filtering the raw water using anozone-resistant membrane, wherein the concentration of ozone present inthe water filtered through the ozone-resistant membrane is detected andan amount of ozone to be fed to the raw water is continuously andautomatically controlled so that the concentration of ozone is adjustedto a prescribed value.

Embodiments of the water treatment process of the present invention asdescribed above include:

a water treatment process comprising detecting a concentration of ozonepresent in the water filtered through an ozone-resistant membrane andcontrolling continuously and automatically an amount of the ozone to befed to the raw water so that the concentration of ozone is adjusted to aprescribed value in the range of 0.05 to 1.0 mg/l;

a water treatment process, wherein the water filtered through theozone-resistant membrane is further treated with activated carbon;

a water treatment process, wherein the water filtered through theozone-resistant membrane is further treated using a reverse osmosismembrane;

a water treatment process, wherein, before the treatment with a reverseosmosis membrane, the filtrate is subjected to aeration or treatmentwith activated carbon, or sodium thiosulfate is added to the filtrate;

a water treatment process, wherein a coagulant is added to raw waterprior to the filtration with an ozone-resistant membrane; and

a water treatment process, wherein a pH value of the raw water iscontrolled to enhance the effect of the coagulant.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a treatment flow of one of the embodiments of the presentinvention.

FIG. 2 shows a treatment flow of another embodiment of the presentinvention.

FIG. 3 shows a treatment flow of a further embodiment of the presentinvention.

FIG. 4 shows a treatment flow of yet another embodiment of the presentinvention.

FIG. 5 is a graph showing a relation between an ozone concentration ofwater filtered through an ozone-resistant membrane and a membranefiltration flux in Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates basically to a water treatment processcomprising adding ozone to raw water and filtering the ozone-containingwater with an ozone-resistant membrane.

FIGS. 1 to 4 are water treatment flow diagrams showing embodiments ofthe present invention.

FIG. 1 shows a basic flow diagram comprising detecting an ozoneconcentration of water filtered through an ozone-resistant membraneusing an ozone concentration measuring apparatus and automaticallycontrolling an ozone amount generated by an ozone generator.

FIG. 2 shows a flow diagram, wherein an amount of ozone to be fed to rawwater is controlled in the water treatment process wherein the waterfiltered through an ozone-resistant membrane is further treated using anactivated carbon treatment device.

FIG. 3 shows a flow diagram, wherein an amount of ozone to be fed to rawwater is controlled in the water treatment process wherein the waterfiltered through an ozone-resistant membrane is further treated with areverse osmosis membrane.

FIG. 4 shows a flow diagram, wherein a coagulant is added to raw waterin the process of treating water with activated carbon as shown in FIG.2 or in the process of treating water using a reverse osmosis membraneas shown in FIG. 3.

The term “raw water” used in the present invention means water which issubjected to the advanced treatment of service water and secondarytreatment waste water and the waste water treatment, e.g., river water,lake water, marsh water, ground water, reservoir water, secondarytreatment waste water, industrial waste water and the like.

Conventionally, if raw water as above exemplified is filtered with amembrane as it is, suspended substances contained in the raw water andorganic substances being larger than the pore diameter of the membraneis blocked at the membrane so that a so-called concentrationpolarization or a cake layer is formed and, at the same time, theorganic substances in the raw water clog the membrane or attach to thenetting constructed inside of the membrane. As a result, the membranefiltration flux upon filtering raw water is decreased to between 1 inseveral and 1 in several tens of that upon filtering pure water.

However, if the above raw water is filtered through a membrane in thepresence of oxidizing agents such as ozone, it passes through themembrane while decomposing the organic substances attaching to orclogging the membrane so that an extremely large filtration flux can beachieved. Namely, in the presence of ozone, ozone repeatedly attacks theorganic substances attached to the filtration membrane when passingthrough it. This means that the membrane always carries outself-cleansing during the filtration. As a result, a large filtrationflux can be obtained.

By adding ozone to raw water, there can be expected effects such asimprovement of water quality, i.e., removal of color and foul odor andtaste, and effective removal of iron and manganese during membranefiltration, which is brought by oxidization thereof, besides the effectthat a large filtration flux is obtained as above described. In thepresent invention, it is necessary to add ozone in such an amount thatachieves the above effects at the same time.

Although an amount of ozone added to enhance the water quality dependson the nature of the raw water and the purpose of water qualityimprovement, it is added in an amount so that the ozone concentration is0.05 to 30 mg/l. An excessive amount of ozone causes many by-productsdue to the oxidative destruction by ozone so that the load at thefollowing step of absorbing by-products becomes unpreferably large ormanganese in the raw water is not effectively removed during membranefiltration because of peroxidization thereof. In addition, suchexcessive ozone is not advantageous in economy since ozone increasinglyremains. On the other hand, too small an amount of ozone cannot improvethe water quality sufficiently.

The relation between the amount of ozone fed into raw water and themembrane filtration flux also depends on the quality of raw water. Thepresent inventors have found that even if the same amount of ozone isfed into the same raw water, the membrane filtration flux differsdepending on whether or not ozone remains on the surface of a filtrationmembrane, and that the filtration flux becomes larger in the case thatozone remains on the surface of the filtration membrane since themembrane surface is cleansed all the time.

Then, the present inventors made studies on the relation between theozone concentration and the membrane filtration flux under the conditionthat ozone remains in the raw water (i.e., on the surface of thefiltration membrane). As a result, they discovered that the ozoneconcentration and the membrane filtration flux are in the relation asshown in FIG. 5. Namely, when the amount of the residual ozone infiltrate is 0.3 mg/l or more, the membrane filtration flux is almostconstant. When it is less than 0.05 mg/l, the membrane filtration fluxgreatly depends on the ozone concentration of the filtrate and changessharply.

In other words, it has been found that when the ozone concentration offiltrate is 0.3 mg/l or more, further increase of the membranefiltration flux cannot be expected, and that when the ozoneconcentration of filtrate is less than 0.05 mg/l, it is difficult toobtain the filtrate in a constant amount due to considerable changes inthe membrane filtration flux.

On the other hand, the present inventors also found that if ozone is fedinto raw water so that the ozone concentration of filtrate falls in therange of 0.05 mg/l or more and less than 1.0 mg/l, water quality issufficiently improved.

Considering the above facts, for the purpose of achieving a necessaryand sufficient ozone treatment effect according to the filtrationprocess comprising adding ozone to raw water and filtering it with anozone-resistant membrane, the amount of ozone fed into raw water iscontrolled so that the concentration of ozone present in filtrate is aprescribed value in the range from 0.05 to 1 mg/l, preferably in therange from 0.05 to less than 0.3 mg/l.

Hereinafter, each step of the process is described in more detail.

Control of Ozone Amount

The present invention has a feature that an amount of ozone to be fedinto raw water is continuously and automatically controlled so that theozone concentration of filtrate is a prescribed value when raw water isfiltered through an ozone-resistant membrane after the addition ofozone.

As shown in the flowcharts of FIGS. 1 to 4, raw water 1 is treated byozone in step 2 of ozone treatment and is filtered through anozone-resistant membrane in step 3. The ozone concentration of the waterfiltered through the membrane is measured by the ozone concentrationmeasuring apparatus 4 all the time. Based on the measured values, theamount of ozone supplied from an ozone generator 5 in the ozonetreatment step is automatically controlled so that the concentration ofthe residual ozone falls in, for example, the range from 0.05 to lessthan 0.3 mg/l.

As the ozone concentration measuring apparatus 4, a ultravioletabsorbance method, an electrode method, an iodometric method, an indigoblue method, a fluorescence method, a coloring method and the like canbe employed. Among them, a method wherein feedback control is exercisedwith high accuracy in a short time is preferable. The ultravioletabsorbance method and the electrode method are preferable since they canarithmetically calculate a measured value. The measured value by theozone concentration measuring apparatus 4 is computed with a CPU(central processing unit) and transmitted to the ozone generator 5. Inthe ozone generator 5, a concentration of ozone to be generated isincreased or decreased by means of controlling current or voltage of theapparatus according to the transmitted signal.

Ozone Treatment

In the step 2 of ozone treatment, the ozone to be added to raw water maybe in the form of ozone units or ozonized air. The ozone can beintroduced into raw water through an air diffuser tube and the likeinstalled in a reaction column arranged in front of a raw water vesselor an air diffuser tube and the like arranged at an appropriate positionin a raw water vessel. Instead of the reaction column equipped with theair diffuser tube, a U-tube system can be also employed.

Further, in addition to the above supplying systems, ozone may be fedfrom an ejecting or line-mixing system which is set in the middle of atube introducing raw water to the ozone-resistant membrane.

When ozone is generated by electrical discharge, the raw material forgenerating ozone may be either air or oxygen. Further, ozone generatedby electrolysis of water is acceptable. Preferably, ozone is suppliedinto raw water continuously.

By the addition of ozone, microorganisms inhabiting the raw water 1 suchas virus, bacteria, mold and protozoa can be killed. Further, thesuspended substances or organic substances in the raw water 1 aredecomposed by the ozone, and at the same time, the raw water containingozone is filtered while decomposing the organic substances attaching toor clogging the ozone-resistant membrane described below. As a result,an extremely large filtration flux can be achieved. Namely, since theorganic substances adhering to the membrane is repeatedly attacked bythe ozone passing through the membrane, the membrane alwaysself-cleanses throughout the filtration so that a large filtration fluxcan be obtained.

Although special attention is not necessary regarding the contactingperiod of the raw water 1 and ozone, it is generally 1 second to 30minutes.

Membrane Filtration Apparatus

The filtration apparatus used in the present invention should beequipped at least with a vessel or tank for reserving raw water(hereinafter referred to as a raw water vessel), a membrane module,means for transferring raw water to the membrane module (e.g.,circulation pump, etc.), a vessel or tank for reserving water filteredthrough a membrane (hereinafter referred to as a filtrate tank), andmeans for backwashing the membrane. In the case of a cross-flowfiltration system, a line for returning raw water not passing throughthe membrane (circulating water) to the raw water vessel is arranged.

Membrane Filtration Treatment with Ozone-Resistant Membrane

An ozone-resistant membrane to be used in the present invention is notparticularly restricted as long as it is not ruined by ozone. Forexample, there can be employed inorganic membranes such asozone-resistant ceramic and organic membranes such as fluorine typeresin membranes, e.g., a polyvinylidene fluoride (PVDF) membrane, apolytetrafluorinated ethylene (PTFE) membrane, anethylene-tetrafluoroethylene copolymer (ETFE) membrane, and apolyfluoroacrylate (PFA) membrane. In particular, the polyvinylidenefluoride (PVDF) membrane is preferably employed.

Among the above ozone-resistant membranes, one with pore diametersranging from those suitable for an ultrafiltration (UF) membrane tothose suitable for a microfiltration (MF) membrane can be used.Basically, a microfiltration (MF) membrane with a high filtration rateis preferably used. For example, a membrane with an average porediameter of preferably 0.01 to 1 μm, more preferably 0.05 to 1 μm, issuitably used.

The ozone-resistant membrane is shaped optionally in a hollow yarn, aplain membrane, pleats, a spiral, a tubular and the like. Of these, ahollow yarn shaped membrane is preferable since membrane area per unitvolume is large. In general, the filtration is carried out in a moduleinstalling a membrane.

The filtration may be carried out according to either a whole quantityfiltration method or a cross-flow filtration method. In the case of thecross-flow filtration method, air or oxygen contained in the suppliedozone gas is returned to the raw water vessel together with thecirculating water and is separated into gas and liquid. On the otherhand, in the case of the whole quantity filtration method, it isnecessary to remove ozonized air existing as unreacted gas in the rawwater chamber of the membrane module. Some devices to, for instance,arrange a vapor-liquid separation apparatus in the upper part of themembrane module must be invented.

Further, either a pressure filtration method or a negative pressurefiltration method is employed. The pressure filtration method is morepreferable since a larger filtration flux can be obtained. Eitherinternal pressure or external pressure filtration method is employed.

In order to maintain filtration performance of the membrane, themembrane module is regularly subjected to physical cleansing. As thephysical cleansing, backwashing and air-scrubbing are mainly effective.

The backwashing is preferably carried out using the water filteredthrough the ozone-resistant membrane.

The air-scrubbing comprises stopping filtration after operation for acertain period, feeding a gas on the surface of the membrane facing theraw water, and vibrating the membrane surface to cleanse the membrane.In the present invention, since the organic substances attaching to themembrane surface is decomposed by ozone to be non-attaching substances,the non-attaching substances (organic and inorganic substances) cloggingthe pores of the membrane are effectively shaken off by theair-scrubbing. Therefore, great cleansing effects can be achieved.

The air-scrubbing may be combined with the backwashing. They may beconducted in the order of filtration, air-scrubbing and thenbackwashing; filtration, backwashing and then air-scrubbing; orfiltration and then simultaneously air-scrubbing and backwashing.

Further, the air-scrubbing may be conducted either with or withoutrunning raw water at the same time. Or, the air-scrubbing andbackwashing may be conducted alternately.

The air-scrubbing is preferably conducted for 1 second to 6 minutes.When its operation period is less than 1 second, the effect of theair-scrubbing cannot be exhibited. On the other hand, when it is morethan 6 minutes, a pause of the filtration is prolonged so that an amountof filtrate is unpreferably reduced.

Activated Carbon Treatment

In the flowchart of FIG. 2, the activated carbon treatment apparatus 6is an apparatus for removing a very small amount of organic substancescontained in the water filtered through the ozone-resistant membrane,biologically easily decomposable organic substances produced by thereaction with ozone or by-products produced by the reaction with ozoneto produce highly treated water. Specifically, the activated carbontreatment comprises introducing water filtered through theozone-resistant membrane into a tank containing granular activatedcarbons and carrying out post-treatment.

As the activated carbon, biological activated carbon (BAC) is preferablyused. The BAC is particularly effective to remove biologically easilydecomposable organic substances produced by the reaction of humin andthe like with ozone.

When the activated carbon treatment is conducted, it is important tocontrol the ozone concentration of the water filtered through theozone-resistant membrane at a low level for the following reasons. Ifthe ozone concentration in the filtrate is high, the activated carbonsreact with ozone. As a result, oxygen gas is generated and an air lockphenomenon is caused, so that water filtration resistance increases orwater cannot be filtered. The ozone concentration of over 1.0 mg/lconsiderably causes such phenomena. A high ozone concentration puts agreater load on the activated carbon in the activated carbon treatmentstep. In such a case, there is the possibility that microorganisms inthe biological activated carbons may be killed by ozone. The ozoneconcentration in the filtrate is in the range of preferably from 0.05 to1.0 mg/l, more preferably 0.05 to less than 0.3 mg/l, most preferablyfrom 0.05 to 0.25 mg/l.

De-ozonization Treatment

In the membrane filtration treatment with a reverse osmosis membrane asshown in the flowchart of FIG. 3, it is desirable to de-ozonize thefiltrate prior to the membrane filtration treatment 8 with a reverseosmosis membrane for the purpose of protecting the reverse osmosismembrane which does not have a resistance to ozone. For example, thede-ozonization treatment comprises arranging a residence tank andcarrying out aeration to remove ozone from the filtrate, or carrying outtreatment with a reducing agent such as sodium thiosulfate or activatedcarbons to decompose residual ozone.

Membrane Filtration Treatment with Reverse Osmosis Membrane

When water is treated with a reverse osmosis membrane, a treatmentprocess comprising coagulation sedimentation and sand filtrationtreatment and the like are generally performed as a pre-treatment forremoving suspended substances. According to these methods, it ispossible to reduce the content value, i.e., the fouling index (FI)value, of suspended substances contained in the water subjected to thepre-treatment of the filtration with a reverse osmosis membrane to 3 orless. Such a value, however, is not sufficient enough.

According to the water treatment with an ozone-resistant membrane, whichthe present invention proposes, the FI value can be 1 or less sincesuspended substances and microorganisms in the raw water 1 are blockedby the ozone-resistant membrane. Therefore, in the case that the waterfiltered through the ozone-resistant membrane is further treated with areverse osmosis membrane as shown in the flowchart of FIG. 3, filtratewith uniform quality can be constantly transferred to the reverseosmosis membrane regardless of the changes in quality, amount andtemperature of the raw water 1. Accordingly, no load is put on thereverse osmosis membrane, performance of the treatment is sufficientlymaintained and a large filtration flux can be obtained. A reverseosmosis membrane facility can be downsized. Further, as the supplypressure can be lowered, the energy cost becomes low.

Since the water treatment process of the present invention is structuredbased on the filtration with an ozone-resistant membrane, a filtrationflux becomes large and the filtration is carried out very effectively.As a result, the cost of the whole facility can be reduced.

The membrane filtration treatment 8 with the reverse osmosis membranecan, for example, remove organic substances with high hydrophilicitysuch as polysaccharide which are not digested by microorganisms and aredifficult to attach to the activated carbons according to the biologicalactivated carbon treatment. Further, when the water temperature islowered in the biological activated carbon treatment, the biologicalactivity is reduced and treatment performance is deteriorated. However,the use of a reverse osmosis membrane provides the advantage of lessdependency on temperature.

The type of the reverse osmosis membrane is not particularly limited. Alow-pressure reverse osmosis membrane and a nano-filter can be usedbesides common reverse osmosis membranes. If the low-pressure reverseosmosis membrane and the nano-filter, which are suitable for lowpressure treatment, are used, a filtration operation pressure can beincreased so that a filtration flux is preferably enlarged.

The reverse osmosis membrane can obstruct soluble organic substances,micro-pollutants such as agricultural chemicals, and inorganic salts.Therefore, it is effective to obtain drinking water or industrial waterfrom highly contaminated raw water or raw water having a high saltconcentration.

Addition of Coagulant

When a membrane having a pore diameter encompassed within amicrofiltration (MF) area, particularly a membrane having a large porediameter, is used for the filtration with an ozone-resistant membrane,suspended substances (SS), bacteria and the like contained in the rawwater 1 invade in the membrane. Particularly, the membrane cloggingcaused by highly viscous substances is extremely difficult to wash offby means of a common membrane filtration operation.

For the membrane filtration using a microfiltration (MF) membrane with alarge pore diameter, coagulants such as polyaluminum chloride (PAC),aluminum sulfate, ferrous chloride and ferric chloride are preferablyadded to the raw water. A flowchart in the case of using coagulants isexemplified in FIG. 4.

The coagulant may be added in a reservoir tank such as a tank forreserving the raw water 1, or in the middle of a tube transferring theraw water 1 to a chamber where ozone is added, or may be added by meansof line-mixing method in the middle of a tube transferring the raw water1 to the ozone-resistant membrane after the addition of ozone.

For the purpose of further improving the effects of the coagulant, thepH value of the raw water may be controlled to desired levels by liquidchemicals and the like. An appropriate pH value differs depending on thetype of the coagulant. It is adjusted so as to be in the range of 2 to8, preferably 2 to 7.5 before, during or after the addition of thecoagulant.

The liquid chemicals for adjusting the pH value are added according tothe same manner as used for the addition of the coagulants (e.g.,addition to the raw water tank and line-mixing addition) prior to orsimultaneously with the addition of the coagulants. The pH value issuitably adjusted by mineral acids such as hydrochloric acid, sulfuricacid and nitric acids when the raw water is alkaline, and by sodiumhydroxide, potassium hydroxide or the like when the raw water is acid.

If a coagulant is used together with the liquid chemicals, organicsuspended substances or polymeric substances are coagulated and areunlikely to contact with ozone. As a result, it can be expected to halvethe necessary amount of ozone.

The coagulants need to be added in such an amount that is capable ofcoagulating the suspended substances contained in the raw water 1. Ingeneral, such an amount is preferably 1 to 100 mg/l, more preferably 2to 50 mg/l of the raw water 1.

Hereinafter, the Examples of the present invention is described.

EXAMPLE 1 Present Invention

As the raw water 1, river surface water with turbidity of 3 to 4degrees, chromaticity of 5 to 10 degrees, COD (chemical oxygen demand)of 6 to 8 mg/l, and water temperature of 12° C. was employed. As shownin FIG. 1, water treatment was carried out in the sequence of raw water1→ozone treatment 2→membrane filtration 3 with an ozone-resistantmembrane. The ozone concentration of the water filtered through theozone-resistant membrane was measured with an ozone concentrationmeasuring apparatus 4. In order to keep the ozone concentration at aprescribed value, an amount of ozone generated from an ozone generator 5is automatically increased or decreased by a CPU.

As an ozone-resistant membrane used for the membrane filtrationtreatment 3 with an ozone-resistant membrane, a microfiltration (MF)membrane in the form of a PVDF (polyvinylidene fluoride) hollow fiberwith an average pore diameter of 0.1 μm, which was prepared according toJapanese Patent Application Laid-Open No. 3-215535 was employed. Themodule used was an external pressure type module comprising installing abundle of 1,800 PVDF hollow fiber membranes with an internal diameter of0.7 mm and an external diameter of 1.25 mm in a PVC (polyvinyl chloride)casing with a diameter of 3 inches. When a membrane area was 7.0 m² anda module filtration pressure was 50 kPa, a pure water flux was 1.8m³/hr.

For the filtration, the cross-flow system was employed. The raw water 1was fed into a raw water vessel, and was further supplied to theabove-described PVDF hollow fiber module at a constant incoming pressurewith a pump to carry out constant pressure filtration. The amount ofcirculation water was controlled so that the ratio of the filtrateamount to the circulation water amount may be 1 to 1.

Between the exit of the pump and the module, an ejector type ozonesupply port was arranged. Ozone produced using air as a raw material wasfed therefrom.

The treatment was carried out by repeating an operation comprising 10minute filtration followed by 15 second backwashing with filtrate.During the treatment, air-scrubbing was carried out for 120 secondsevery 12 hours by supplying air in an amount of 2 Nm³/hr from the bottomof the module.

The value of a signal controlling an amount of ozone to be generated,which was transmitted from the ozone concentration measuring apparatus 4to the ozone generator 5, was set so that the ozone concentration of thewater filtered through the ozone-resistant membrane was a prescribedvalue. Then, the above filtration treatment was performed for 50 hours.A membrane filtration flux after 50 hours was measured, and was dividedwith the pure water flux obtained under the same filtration pressure.The result shown in FIG. 5 was obtained.

The filtrate was analyzed. As a result, it was found that when the ozoneconcentration of the filtrate was 0.05 mg/l or more, the turbidity was0.02 degrees, the chromaticity was 2 degrees or less and colon bacillusor general bacteria were not detected at all. On the other hand, whenthe ozone concentration of the filtrate was less than 0.05 mg/l, theturbidity was 0.02 degrees, and neither colon bacillus nor generalbacteria were detected, but the chromaticity was 5 to 7 degrees.

EXAMPLE 2 Present Invention

In Example 1, the operation was carried out so as for the ozoneconcentration of the filtrate to be 0.2 mg/l, and the filtrate wastransferred to an activated carbon vessel (see the flowchart of FIG. 2).As the activated carbon, F400 manufactured by Calgon Co., Ltd. wasemployed. The operation was designed so as for the EBCT (Empty BedContact time) to be 20 minutes.

As a result, the water quality analysis, the water filtered through theozone-resistant membrane had a turbidity of 0.02 degrees, a chromaticityof 2 degrees or less, and a COD of 4 to 5.5 mg/l. Further, the qualityof the water passed through activated carbon was a turbidity of 0.02degrees, a chromaticity of 1 degree or less, and a COD of 0.3 to 0.8mg/l.

EXAMPLE 3 Present Invention

As the raw water 1, secondary treatment waste water having a turbidityof 5 to 11 degrees, a chromaticity of 18 to 20 degrees, a COD of 20 to30 mg/l, a pH value of 7.2 to 7.6, and a temperature of 23° to 25° C.was employed. As well as Example 1, the raw water was treated in thesequence of raw water 1→ozone treatment 2→membrane filtration 3 withozone-resistant membrane as shown in FIG. 1. The operation was designedso that the ozone concentration of the water filtered through theozone-resistant membrane was 0.25 mg/l.

The quality of the filtrate was a turbidity of 0.1 degree or less, achromaticity of 2 degrees or less, and a COD of 6 to 8 mg/l. Further,when a part of the filtrate was collected and residual microorganismswere detected according to a provisional examination method fordetecting agar mediums and Cryptosporidium oocyst, the existence of suchmicroorganisms was not confirmed at all.

A part of the concentrated water was also collected and examined. Theexistence of dangerous residual and alive microorganisms was notdetected. The concentrated waste water was also confirmed to be safe.

EXAMPLE 4 Present Invention

Using the same raw water and ozone-resistant membrane module as inExample 3, the water treatment process with the reverse osmosis membraneas shown in FIG. 4 was carried out. Namely, the raw water was treated inthe sequence of raw water 1→pH control 9→addition of coagulant 10→ozonetreatment 2→membrane filtration 3 with ozone-resistant membrane. A partof the obtained filtrate was subjected to the de-ozonization treatment 7and then was subjected to the membrane filtration treatment 8 with areverse osmosis membrane.

The pH value was controlled so as to be 6.2 to 6.5 by arranging a staticmixer on a line transferring raw water to a raw water tank (which is notshown in the figure) in the membrane filtration apparatus using anozone-resistant membrane and adding sulfuric acid thereto. Further,between the static mixer and the raw water vessel, another static mixerwas arranged to add ferric chloride (FeCl₃) as a coagulant in an amountof 35 mg/l of the raw water. Then, an ozone treatment was carried out.

The ozone concentration of the water filtered through an ozone-resistantmembrane was detected using the ozone concentration measuring apparatus4, and was controlled so as to be 0.05 mg/l by automatically decreasingor increasing the ozone amount generated from the ozone generator 5through a CPU.

The quality of the water filtered through the ozone-resistant membranehad a turbidity of 0.1 degree or less, a chromaticity of 1 degree orless, and a COD of 4 to 6 mg/l.

The de-ozonization treatment was carried out by adding sodiumthiosulfate in an amount of 0.15 mg/l of raw water to the water filteredthrough an ozone-resistant membrane and decomposing residual ozone.

Next, a part of the resultant treated water was supplied to an aromaticpolyamide type compound membrane spiral nano-filter. The aromaticpolyamide type composite membrane spiral nano-filter had a NaCl blockingrate of 65%, MgCl₂ blocking rate of 50% and sucrose blocking rate of99%.

The above-mentioned aromatic polyamide type composite membrane spiralnano-filter was arranged into two-step cascade, and was driven for 2months at a filtrate recovery rate of 70% under a filtration pressure of40 kPa. The operation was constantly carried out all through, and afiltration amount of 5 m³/day was obtained. In addition, a TOC (totalorganic carbon) removal rate was kept at 90 to 97% throughout theoperation. The quality of the resultant treated water was high enoughfor subjecting to reuse.

EXAMPLE 5 Present Invention

Using the raw water employed in Example 1, the filtration was carriedout according to the whole quantity filtration system under a constantpressure. Namely, water was supplied to the raw water vessel in the sameamount as the filtrate, and a vapor-liquid separation apparatus wasarranged at the upper part of the membrane module. The ozone additionmethod, ozone-resistant membrane employed and operation conditions werethe same as Example 1.

The operation was carried out so as for the ozone concentration of thefiltrate to be 0.25 mg/l. As a result, the value obtained by dividingthe membrane filtration flux after 50 hour operation with the pure waterflux under the same membrane filtration pressure was 79%, which was thesame value as the cross-flow system filtration. Further, the quality ofthe filtrate was the same as the cross-flow system filtration. Nodifference was observed between the whole quantity filtration system andthe cross-flow filtration system.

Industrial Applicability

According to the present invention, filtrate with a certain high qualitycan be constantly produced in the process of membrane filtrationtreatment of water using ozone while maintaining a large filtration fluxregardless of changes in the quality of raw water.

Further, according to the present invention, the amount of ozoneremaining in the filtrate can be kept small during the process ofmembrane filtration treatment of water using ozone so that the followingwater treatment can be carried out effectively.

Furthermore, according to the present invention, treated water with ahigh quality is obtained in a compact system, and a water treatmentprocess without producing harmful wastes can be achieved.

Consequently, according to the present invention, treated water with agood quality can be economically and constantly produced in the advancedtreatment of service water and secondary treatment waste water,treatment of industrial water or waste water, and the like.

What is claimed is:
 1. A water treatment process comprising adding ozoneto a raw water and filtering the raw water using an ozone-resistantmembrane, wherein a concentration of ozone present in water filteredthough the ozone-resistant membrane is detected and an amount of ozonefed to the raw water is continuously and automatically controlled so asto adjust the concentration of ozone in the filtrate to a prescribedvalue, and wherein the amount of the ozone fed to the raw water isadjusted so that the concentration of ozone present in the filtrate isat said prescribed value, which prescribed value is in the range of 0.05to 0.3 mg/l.
 2. The water treatment process according to claim 1,wherein the filtrate is further treated with activated carbon.
 3. Thewater treatment process according to claim 1, wherein the filtrate isfurther treated with a reverse osmosis membrane.
 4. The water treatmentprocess according to claim 3, wherein, before the treatment with thereverse osmosis membrane, the filtrate is subjected to aeration ortreatment with activated carbon, or sodium thiosulfate is added to thefiltrate to remove residual ozone.
 5. The water treatment processaccording to claim 1, wherein a coagulant is further added to the rawwater.
 6. The water treatment process according to claim 5, wherein,before, after or at the same time as the addition of the coagulant, a pHvalue of the raw water is adjusted to 2 to 8.